[Improved thyroid diagnosis in borderline hyperthyroidism by determination of free T3 index]

The measurements of circulating total thyroxine (T4) and total triiodothyronine (T3) depend on the concentrations of thyroid hormone binding proteins, especially TBG. A simple approach, the calculation of free T3 index (FT3I) and augmented FT3I (aFT3I) from total T3 and T3-resin uptake, corrects the total serum T3 for variation in thyroid hormone binding capacity. In a group of 95 patients with borderline elevated T3 levels (3.0-4.5 nmol/l) we have correlated the results of FT3I and aFT3I with clinical, biochemical and radioisotope findings: in a subgroup of 23 euthyroid females taking oestrogens (group A1) and in 22 other patients with TBG elevations (independent of oestrogens, group A2) the results for total T3 were within the slightly hyperthyroid range (3.32 +/- 0.3 and 3.26 +/- 0.24 nmol/l respectively), but FT3I was always within the normal range. In contrast, FT3I was clearly elevated in 50 hyperthyroid patients (group B) compared to the total T3 (4.6 +/- 0.6 and 3.9 +/- 0.4 nmol/l respectively). An even better discrimination was obtained by calculation of augmented FT3I. It is concluded that in patients with moderate elevations of T3 the FT3I and aFT3I are capable of distinguishing clearly between the euthyroid and hyperthyroid range. The estimation is thus indicated in all patients with changes in thyroid hormone binding protein concentration and is of great value in confirming or ruling out the diagnosis of T3-toxicosis, T4-toxicosis and in all cases of "preclinical" or borderline hyperthyroidism.